Any former mayor of a municipality having a population of not fewer than twenty-one thousand eight hundred (21,800) nor more than twenty-two thousand eight hundred (22,800) according to the 1990 Federal Decennial Census who is receiving a monthly retirement annuity of two hundred fifty dollars ($250) from a municipality shall receive an increase to three hundred dollars ($300) per month effective July 1, 1999. Thereafter, the municipality shall redetermine the amount of the monthly benefit each January 1. The redetermined amount shall be payable for the following twelve (12) calendar months. The redetermined amount shall be the amount of the benefit payable as of the immediately preceding December 1 increased by three percent (3%).